1. Field of the Invention
The present invention relates generally to a wireless communication system. More particularly, the present invention relates to an apparatus for bidirectional communication using one or more auxiliary bands in a wireless communication system.
2. Description of the Related Art
For bidirectional communication, a wireless communication system uses a duplex scheme capable of discriminating between downlink (DL) transmission and uplink (UL) transmission. Referring to FIGS. 1A and 1B, which illustrate conventional communication systems, the duplex scheme can be classified into a frequency division duplex (FDD) scheme (FIG. 1A) and a time division duplex (TDD) scheme (FIG. 1B). The FDD scheme discriminates between a DL 101 and a UL 103 by allocating different frequency bands to the DL 101 and the UL 103. The TDD scheme discriminates between a DL 105 and a UL 107 by allocating different transmission intervals to the DL 105 and the UL 107 while allocating the same frequency band thereto.
In the case of the FDD scheme, a guard band must be inserted between a DL band and a UL band in order to prevent DL-UL interference. Also, because a DL-UL ratio is fixed by the bandwidth, the FDD scheme is unsuited to accommodate a variable DL-UL asymmetric traffic. The FDD scheme is used in the 2G IS-95 system, the Global System for Mobile communications (GSM) system, and most of the 3G systems, which provide voice-oriented services. However, the FDD scheme is unsuitable for wireless communication systems providing data-oriented services.
The TDD scheme can effectively cope with an asymmetric traffic by flexibly adjusting a DL-UL ratio. Also, because the DL and UL links use the same frequency band, the TDD scheme can reduce the overhead necessary for feedback information using channel reciprocity. Thus, the TDD scheme enables the effective use of technologies which increase frequency use efficiency, such as adaptive modulation and multiple antenna techniques.
However, the TDD scheme is more complex than the FDD scheme in terms of synchronization. Also, a guard interval corresponding to a propagation delay and a switching delay must be inserted into a DL-UL transition gap, which causes an overhead in transmission of user data.
Furthermore, the frame length must be increased in order to reduce the overhead due to the guard interval and frame-by-frame control signals such as sync signals. However, when the frame length is increased, an OFF interval of each of DL and UL and a delay of control signals such as channel information and feedback information for a retransmission request are increased. Therefore, in the case of user data requiring a small delay and user data of high-speed movement, the performance degradation occurs due to a delay of the control signal.